use default image for test

0212.node_worker_bwforcluster_container.cfg

@@ -97,7 +97,7 @@ SINGULARITY = /usr/bin/apptainer
 #  "/cvmfs/unpacked.cern.ch/registry.hub.docker.com$(docker_image)")
 # SINGULARITY_IMAGE_EXPR = "/cvmfs/unpacked.cern.ch/registry.hub.docker.com/mschnepf/slc7-condocker:latest"
 SINGULARITY_HOOK_PREPARE_JOB = $(ETC)/git/scripts/prepare_singularity.py
-SINGULARITY_IMAGE_EXPR = TARGET.ContainerImage
+SINGULARITY_IMAGE_EXPR = ifThenElse(isUndefined(TARGET.ContainerImage), /cvmfs/unpacked.cern.ch/registry.hub.docker.com/mschnepf/slc7-condocker:latest, TARGET.ContainerImage)
 SINGULARITY_TARGET_DIR = /srv
 MOUNT_UNDER_SCRATCH =
 # The container needs to use the execute directory as HOME directory